Chapter 8 The UNIVAC system 167

and realize performance in any large scale and complex apparatus of the same character.

Some of the troubles encountered are interesting to study in detail. On a rather complicated routine requiring the use of a number of Uniservos, all ran smoothly for 15 minutes. At that time, one of the Uniservos executing a backward read somewhere in the middle of the reel, did not stop at the end of the block hut continued to run until it ran off the end of the tape. After much work, it was shown that a cycling unit signal was being overloaded because it was being used both by a multiplication instruction and the backward read which were occurring simultaneously. The input precessor loop was cleared as a result and the count of the pulses coming off the tape was thereby lost. Once the trouble was found, it was simple to remedy.

Another rather interesting case occurred intermittently over an extended period. Normally when reading out of the memory, the contents should not be cleared. Occasionally, however, reading from the memory also caused the contents to be cleared. As the trouble only remained for a period of seconds or, at most, a few minutes, it was somewhat difficult to localize. Of course, parasitic oscillations of some sort were suspected and, in fact, the trouble was traced to the actual source on a logical basis; but the source, a high power cathode follower, showed no evidence of oscillation. Before the problem was remedied, various combinations of parasitic suppressors were tried; the trouble would vanish for perhaps a week and then return. The oscillation finally cropped up during a maintenance shift, was found to be in the suspect tube at 100 megacycles and was eliminated rather easily.

Other types of troubles that have occurred include intermittent parasitic oscillations in other circuits, bounce in Uniservo relay circuits, various mechanical problems in Uniservos, time constants not consistent with the longest duty cycle signals, and various types of noise in the input circuits. The tubes, which initially were bothersome, have now stabilized to the point where two tubes per week (on the average) stop the computer during computation.

All of the above troubles and others not discussed here have contributed to lost computing time on the UNIVAC. However, they cannot influence future operation because the reasons for them have been found and eliminated. The fact that these troubles will not occur in future UNIVACs cannot be emphasized too strongly.

Under a contract with the Bureau of Census, Eckert-Mauchly Computer Corporation maintains the Census installation. This system is operated 24 hours a day, seven days a week, except for four 8-hour preventive maintenance shifts each week. This allows approximately 32 hours for regular maintenance and 136 hours for operation or 21 and 79 per cent respectively. Table 1 shows the engineering time spent on the computer system during typical weeks of operation. The figures are given both in hours and percentages. Both nonscheduled engineering time as well as preventive maintenance time are shown. The sum of the two gives the total engineering time spent on the computer per week. It should be noted that this is actual engineering time and does not include time that the computer may have been shut down while waiting for an engineer to report. According to our maintenance contract, this must be within a half hour during regular working hours and within two hours at all other times. Attention should be given to the fact that the preventive maintenance time does not total exactly 32 hours each week. This is due in part to a half-hour period each morning devoted to checking and cleaning the mechanical portions of Uniservos. It is expected that this work will be taken over by the UNIVAC operators since the procedures and the techniques involved are quite simple.

In addition, one extra shift was required the week ending June 3 and three extra shifts the week ending October 7, 1951. These shifts were required to incorporate engineering changes which had been developed over a period of time and could not be incorporated in the equipment during the normal preventive main-